1. Field of the Invention
The subject of the present invention is a device for agitating a liquid in a reactor and for injecting a gas into this liquid, comprising a drive device arranged above the reactor, provided with a vertical output shaft equipped at its end with at least one axial-flow mobile assembly immersed in the liquid.
2. Description of the Related Art
The gas injected into the liquid can be either an oxygenated gas with a proportion of oxygen varying from 20 to 100%, or carbon dioxide, or an ozonized gas, or a biogas etc. The liquid into which the gas is to be injected is arranged in reactors used in particular for the biological treatment of industrial effluents and whose height generally varies from 2 to 10 meters in depth.
In what follows "reactor" signifies both a natural "basin" (lagoon, pond, lake etc.) and also an enclosed or open headspace "tank" whose walls may be relatively close together or far apart. The reactors into which these systems enable gases to be injected generally contain activated sludges. These reactors may therefore be either natural basins, or open headspace reactors with closely spaced walls, or enclosed reactors, which may or may not be pressurized.
In the field of the biological treatment of water, various types of systems are known depending on whether the gas is injected at the surface, or at the bottom of the basin. For example, there are surface turbines, brushes enabling air to be transferred into the liquid by creating agitation. Such devices can only be used for small heights of water and have limited oxygenation capacities.
Thus Patent EP-0 583 509 from PRAXAIR Technology Inc. describes a system characterized chiefly by a propeller situated in a hollow shaft and entraining, during its rotation and by vortex effect from the surface of the liquid, gas and liquid located under an immersed cover. The gas/liquid mixture thus formed is propelled downwards. The gas bubbles being undissolved rise within a radius of action corresponding overall to that of the cover where they are collected so as to be reinjected again. The inputting of feed gas and venting, as well as the optimal level of the liquid in the cover, are regulated by the pressure prevailing under the cover.
Although the stated transfer efficiencies are very good, the limits of this system are chiefly:
the zone of action limited to a radius much like that of the cover and to a relatively small depth of water, PA1 the enrichment of the gaseous phase with CO2, N2 and other gases arising from the biological activity, in the case of activated sludge applications, and the need to carry out venting causing losses of O2, PA1 the complexity of the pressure regulation under the cover, PA1 use of a gas at high pressure: need to use a booster following a VSA or MPSA (on-site production unit operating by adsorption under pressure or with vacuum regeneration).
Porous "bottom" systems with injection of gas and agitation means are also known. These systems have a fixed and limited oxygenation capacity, and tend to clog. Finally, there are other "bottom" devices comprising jets or the device known by the brand name "Ventoxal" with injection of gas (oxygen-enriched air, or pure oxygen). The "Ventoxal" system developed by the applicant consists of a pump, a Venturi type injection system, a flow distributor and an ejector/nozzle pair, for which the gas injection pressure depends on the height of water and remains chiefly greater than 1.5 bar absolute. The agitation obtained is satisfactory in the bottom of the basin but may be average in the remainder of the volume.